Conductive material and method for filling via-hole

ABSTRACT

The conductive material comprises a first metal material having a melting point of not more than 250° C. and a second metal material having a melting point of not less than 500° C., and is paste at a temperature not more than 250° C. Whereby the conductive material can have much higher conductivity than the resin paste. The conductive material can be used in paste, whereby the conductive material can be buried in the via-hole in the same way as the resin paste.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2002-014470, filed in Jan. 23,2002, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a conductive material and a method forfilling a via-hole, more specifically a conductive material of highconductivity which can fill micronized via-hole without generating void,and method for filling a via-hole suitable for such the conductivematerial.

In electronic circuit boards and semiconductor devices, conductors areburied in via-holes formed in insulating films, for connectinginterconnection layers on different levels.

As methods for filling conductors in such via-holes, conventionallymethod for growing conductor in a via-hole by electroplating and methodfor filling in a via-hole a conductive resin paste containing metalpowders dispersed in an organic component have been used.

However, with the recent micronization of electronic circuit boards andsemiconductor devices, diameters of via-holes are so micronized that theconventional methods for filling the via-hole are insufficient to fillsuch via-holes.

Specifically, in the method for filling the via-hole by electroplating,as shown in FIG. 3A, usually a plated film 106 starts to grow on thesidewall and the bottom surface of the via-hole 104. At the upper endedge, where an electric field is concentrated, and a current density ishigher, a growth rate is increased in this region. Accordingly, beforethe inside of the via-hole 104 is completely filled with the plated film106, the upper part of the via-hole 104 is closed, leaving a void 108 atthe center of the via-hole 104 (FIG. 3B). The generation of the void 108becomes significant as the via-hole 104 has a larger aspect ratio.Resultantly, defective conduction and resistance increases often takeplace, and the voids are often expanded by thermal treatment to therebybreak device structures.

As an improvement of this method, a method for suppressing the growth ofa plated film from the sidewall of the via-hole to fill the via-holeonly by the growth of the plated film from the bottom surfaces is beingstudied. However, this method makes no void in the via-hole, but theplating takes much time, which results in very high costs.

The filling method using the resin paste is advantageous in costs to thefilling method by the plating. However, the resin paste, which ismixtures of the organic component and the metal powders, has problems ofhigh conduction resistances, residual gas components, low usable limittemperatures, etc.

As described above, it cannot be said that the conventional methods forfilling the via-hole have been sufficient to fill micronized via-holes.A technique for filling micronized via-holes with a material of highconductivity at low costs without generating voids has been required.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a conductive materialof high conductivity which can fill micronized via-holes at low costswithout generating voids.

Another object of the present invention is to provide a method forfilling a via-hole which is suitable for such the conductive material.

According to one aspect of the present invention, there is provided aconductive material comprising a first metal material having a meltingpoint of not more than 250° C. and a second metal material having amelting point of not less than 500° C., and being paste at a temperatureof not more than 250° C.

According to another aspect of the present invention, there is provideda method for filling a via-hole comprising the steps of: in an equipmenthaving a first pressure, supplying a conductive material to a substratewith a via-hole formed in to bury the conductive material in an upperend part of the via-hole; and changing the pressure in the equipment toa second pressure which is higher than the first pressure to therebydraw the conductive material buried in the upper end part of thevia-hole into the via-hole.

According to the present invention, the conductive material contains thefirst metal material of a melting point of not more than 250° C. meltingpoint and the second metal material of a melting point of not less than500° C., and is paste at a temperature of not more than 250° C., wherebythe conductive material can have much higher conductivity than the resinpaste. The conductive material can be used in paste, whereby theconductive material can be buried in the via-hole in the same way as theresin paste.

In an equipment having a first pressure, the conductive material is fedinto a via-hole formed in a substrate to be buried in upper end part ofthe via-hole, the pressure in the equipment is changed a second pressurewhich is higher than the first pressure, whereby the conductive materialburied in the upper end part of the via-hole is drawn into the via-holeto thereby fill the conductive material in the via-hole. Even micronizedvia-hole can be filled without generating voids.

BRIEF DESCRIPTION FO THE DRAWINGS

FIGS. 1A-1C and 2A-2D are sectional views explaining the method forfilling the via-hole according to one embodiment of the presentinvention.

FIGS. 3A and 3B are sectional views explaining the problems of theconventional method for filling the via-hole.

DETAILED DESCRIPTION OF THE INVENTION

The conductive material according to the present invention contains ametal material A of a melting point of not more than 250° C. and a metalmaterial B of a melting point of not less than 500° C., and is a pasteat a temperature of not more than 250° C.

Here, the conductive material according to the present inventioncontains the metal material A of a melting point of not more than 250°C. so that the conductive material becomes paste when the conductivematerial is filled in the via-hole. The conductive material according tothe present invention is filled in the via-hole by the method which willbe described later at about 250° C. at highest. For the conductivematerial to be paste at this temperature, the metal material A isliquid, i.e., a melting point of the metal material A must be not morethan 250° C.

Preferably a melting point of the metal A is suitably selected inaccordance with a substrate temperature, etc. in a process using theconductive material. In consideration of workability it is preferable touse a metal material of a single element or an alloy material which isliquid at the room temperature to about 100° C. For example, an alloy oflow melting point, such as In-76% Ga alloy, which is liquid at the roomtemperature, 12% Sn-18% Pb-49% Bi-21% In, whose melting point is 58° C.,16% Sn-33% Bi-51% In, whose melting point is 61° C., 47% Sn-57% Bi-26%In, whose melting point is 79° C., 22% Sn-28% Pb-50% Bi, whose meltingpoint is 96-110° C., etc.

As the metal material A, it is preferable to use an alloy containingbismuth. The usual metal has a property of expanding the volume withtemperature increases, but bismuth has a property of shrinking withtemperature increase. The metal material A contains bismuth so that thevolume expansion of the other metal due to temperature increase can beabsorbed by bismuth, whereby the volume expansion of the conductivematerial as a whole can be suppressed.

The conductive material according to the present invention contains themetal material B whose melting point is not less than 500° C. so thatthe metal material B is dispersed in the liquid metal material A to formthe paste conductive material. To this end it is necessary to dispersethe metal material B in a state of powder into the metal material A.

The metal material B can be a metal which can be pulverized and does notmelt at a melting point of the metal material A. For example, a singlemetal powder as of Cu, Ag, Zn, Ni, Fe, Pd, Au, Pt or others, an allypowder as of Au—Sn, Cu—Sn, Fe—Ni or others, or a composite metal powderas of Ag plated Cu powder, Au plated Ni powder or others can be used.The surface oxide film degrades wetting property with respect to themetal material A. It is preferable that all the metal powders aresubjected to anti-oxidation treatment with fatty acid, etc.

When the melting point of the metal material B is near that of the metalmaterial A, the metal material B melts into the metal material A toincrease a melting point of the conductive material itself, which causesa risk of making difficult to fill the via-hole. From this viewpoint, itis preferable that the melting point of the metal material B is not lessthan 500° C.

The conductive material according to the present invention is made pastefor the following reason. That is, in using the conductive materialaccording to the present invention as a conductive material for fillingthe via-hole, the conductive material formed of the molten metalmaterial A alone cannot fill micronized via-hole due to high surfacetension, and even when the conductive material can be filled in thevia-hole, it is difficult to retain the conductive material in thevia-hole.

The metal material B is added to the metal material A and sufficientlykneaded. The surfaces of the metal material B is wetted with respect tothe metal material A, the metal material A plays the role of vehicles ofthe conductive paste and solder paste, and the composite materialbecomes paste. Thus, the paste can be filled in even micronized via-holein the same way as the resin paste. The metal material A even in themolten state is retained owing to the wetting with respect to the metalmaterial B, thereby settling stable without flowing out after filled inthe via-hole.

An addition ratio of the metal material B for making the conductivematerial paste varies depending on a kind and a particle size of themetal material B but is in the range of about 5-40 wt %.

On the other hand, in the resin conductive paste, the electricconduction is made by the contacts among the particles of a metalpowder, and a total of resistances of the contacts is a conductionresistance of the via-hole. A value is large. On the other hand, theconductive material according to the present invention, which is pastecontaining the metal material B dispersed in the metal material A,becomes a single metal in via-hole, and has low conduction resistanceand can have conductivity equal to that of bulk metal.

Next, the method suitable to fill the conductive material according tothe present invention in the via-hole will be explained with referenceto FIGS. 1A-1C and 2A-2D. FIGS. 1A-1C and 2A-2D are sectional viewsexplaining the method for filling the via-hole according to the presentembodiment.

In the following description, the conductive material according to thepresent invention is filled in the via-hole which are passed through asilicon substrate, but the present invention is not limited to thepresent embodiment.

First, the conductive material according to the present invention,containing powder of the metal material B dispersed in the metalmaterial A is prepared. The conductive material is prepared by heatingthe metal material A up to above a melting point thereof, dispersing,and mixing and kneading a prescribed amount of the powder of the metalmaterial B in the molten metal material A to thereby prepare the pasteconductive material.

Then, a silicon substrate 10 with via-holes 12 for the conductivematerial to be filled in is mounted on a stage 10 of, e.g., a vacuumprinting machine (FIG. 1A). The via-holes 12 are formed through thesilicon substrate 10. A silicon oxide film 14 is formed on the surfacesof the silicon substrate including the inside walls of the via-holes 12.

A conducting layer 22 of, e.g., a 0.5 μm-thick Pt/Ti film is formed onthe stage 20. The conducting layer 22 is used as an electrode.

Next, the stage 20 is heated to a temperature which is higher by 5-10°C. than a melting point of the conductive material.

Then, the conductive material 30 which has become paste by heating isset on the silicon substrate 10. The conductive material is set on thesilicon substrate 10 at the edge where the via-holes 12 are not formed.

Then, the interior of the printing machine is depressurized down to aprescribed pressure.

Next, the conductive material 30 is rubbed into the via-holes 12 bymeans of, e.g., urethane or stainless squeegee 40. Thus, the conductivematerial 30 is buried in the upper end part of the via-holes 12 (FIG.1B-FIG. 1C).

Then, the interior of the printing machine is returned to theatmospheric pressure. At this time, the voids 16 formed in the via-holes12 on the side nearer the stage 20 have a lower pressure than theatmospheric pressure, and the conductive material 30 buried in thevia-holes 12 on the side of the surfaces thereof is drawn into thevia-holes 12 (FIG. 2A).

A pressure in the printing machine is made higher than an internalpressure in the printing machine at the time of burying the conductivematerial in the upper end part of the via-holes, whereby the conductivematerial can be drawn into the via-holes. Accordingly, an internalpressure of the printing machine at the time of rubbing in theconductive material is not essentially the atmospheric pressure.Similarly therewith, an internal pressure of the printing machine at thetime of burying the conductive material in the upper end parts of thevia-holes is not essentially a decreased pressure.

Then, in the same way as described above, the application of theconductive material 30, the depressurized processing, the squeezing(FIG. 2B), and the atmospheric pressure processing (FIG. 2C) arerepeated, whereby the conductive material 30 buried in the via-holes 12is sequentially drawn into the via-holes 12, and finally the via-holes12 are filled with the conductive material 30 (FIG. 2D).

Next, after the conductive material 30 is buried in the via-holes 12,heat processing is performed as required. This heat processing has theeffect of reacting the metal material B with the metal material A tothereby making a melting point of the conductive material high. Thisheat processing permits the device to be used stable at temperatureshigher than an application temperature of the conductive material.

The conductive material is thus filled in the via-holes, whereby theconductive material of high conductivity can be filled in the via-holeswithout leaving voids.

It is possible to use, e.g., a dispenser in place of the squeegee tofeed the conductive material sequentially into the respective via-holes.In this case as well, the application of the conductive material, thedepressurized processing, and the atmospheric pressure processing arerepeated to effectively fill the conductive material into the via-holes.In the case where the dispenser is used, the dispenser having a heatingmechanism is used to thereby apply the conductive material according tothe present invention in paste.

After the conductive material 30 is filled in the via-holes 12, a capplating may be formed on the surface of the via-holes 12.

The conductive material 30 according to the present invention can befilled in the via-holes in the same way as the conventional resinconductive paste by making a temperature for the filling higher than amelting point of the conductive material.

EXAMPLE 1

16 g of Sn, 33 g of Bi and 51 g of In were loaded in a porcelaincrucible, and heated and melted at a temperature of 70-80° C. Then, 10 gof 2-3 μm-diameter Cu powder was added and mixed, and the conductivematerial was prepared.

Then, a silicon substrate having 50 μm-diameter via-holes formed at a150 μm-pitch and having the surface covered with a silicon oxide film isset on the stage of a vacuum printing machine.

Then, the stage of the vacuum printing machine is heated to 70° C.

Next, the conductive material heated in advance to 70° C. is set on theupper side of the substrate (the edge part where the via-holes are notformed), and the printing machine was drawn the vacuum.

Then, when a vacuum level arrived at 0.1 kPa, the conductive materialwas rubbed in by urethane rubber squeegee. Furthermore, the conductivematerial was oppositely squeezed, and then the vacuum level was returnedto the atmospheric pressure at once.

Then, the drawing the vacuum was again performed, and the same operationdescribed above was repeated twice.

The thus-prepared specimen was cleaved, and the cleavage plane waspolished and observed with an SEM. The conductive material was filleddown to the bottoms of the via-holes, and neither voids nor gaps wereobserved.

A conduction resistance of the via was measured and was 25 mΩ/via. Theconduction resistance which is much lower than about 300-400 mΩ/via ofthe typical conduction resistance of the via using the conventionalresin paste could be realized.

EXAMPLE 2

20 g of Ag-coated Cu powder of Φ6-7 μm is added to 100 g of molten metalof 18.8 wt % of Sn, 31.3 wt % of Pb and 50 wt % of Bi having a meltingpoint of 95° C., and agitated and mixed into paste. The conductivematerial was thus prepared.

The conductive material was filled into the via-holes in the same way asin Example 1. In Example 2, the filling operation was made with astainless squeegee with the stage heated to 105° C.

The thus-prepared specimen was cleaved, and the cleavage plane waspolished and observed with an SEM. The conductive material was filleddown to the bottoms of the via-holes, and neither voids nor gaps wereobserved. The conduction resistance of the via was 20 mΩ/via.

EXAMPLE 3

20 g of Cu powder of Φ5 μm is added to 100 g of molten metal of 48 wt %of Sn and 52 wt % of In having a melting point of 117° C., and agitatedand mixed into paste. The conductive material was thus prepared.

The conductive material was filled into via-holes in the same way as inExample 1. In Example 2, the filling operation was made with a stainlesssqueegee with the stage heated to 125° C.

The thus-prepared specimen was cleaved, and the cleavage plane waspolished and observed with an SEM. The conductive material was filleddown to the bottoms of the via-holes, and neither voids nor gaps wereobserved. The conduction resistance of the via was 20 mΩ/via.

1. A method for filling a via-hole comprising the steps of: in anequipment having a first pressure, supplying a conductive material to asubstrate with a via-hole formed in to bury the conductive material inan upper end part of the via-hole; and changing the pressure in theequipment to a second pressure which is higher than the first pressureto thereby draw the conductive material buried in the upper end part ofthe via-hole into the via-hole.
 2. A method for filling a via-holeaccording to claim 1, wherein a temperature of the substrate is raisedto a temperature where the conductive material become paste.
 3. A methodfor filling a via-hole according to claim 1, wherein the step of buryingthe conductive material and the step of drawing the conductive materialare repeated to fill the conductive material in the via-hole.
 4. Amethod for filling a via-hole according to claim 1, further comprisingthe step of: performing heat processing at a temperature higher than amelting point of the conductive material after the conductive materialhas been filled in the via-hole.
 5. A method for filling a via-holeaccording to claim 1, wherein in the step of burying the conductivematerial, the conductive material is rubbed into the substrate with asqueegee.
 6. A method for filling a via-hole according to claim 1,wherein in the step of burying the conductive material, the conductivematerial is poured into the substrate with a dispenser.
 7. A method forfilling a via-hole according to claim 1, wherein the conductive materialcomprises a first metal material having a melting point of not more than250° C. and a second metal material having a melting point of not lessthan 500° C., and is paste at a temperature of not more than 250° C.